1. Field of the Invention
The present invention relates to a rectifier circuit and a radio communication device including the rectifier circuit.
2. Description of the Related Art
A rectifier circuits converts alternating-current (AC) into direct-current (DC) through the rectification of diodes. The rectifier circuit, when is provided as a semiconductor integrated circuit, employs a diode-connected MOS transistor whose source and gate are connected to each other, as a diode. For example, when an NMOS transistor isolated from a substrate through a triple well is used as a diode, its drain and source are connected to an n-well and the source is also connected to a backgate connected to a p-well located at the bottom of the transistor. This diode functions in a PN junction formed between the source and drain.
A radio frequency identification (RFID) tag, which is categorized as a communication device and is recently watched because of its wide application, requires the rectifier circuit. The RFID tag generates the direct-current power-supply voltage for driving the integrated circuit in the RFID tag and demodulates data signals, from an alternating-current induced in a loop antenna. The voltage generation and demodulation require the rectifier circuit.
Such a rectifier circuit used in the RFID tag is proposed in, for example, Japanese Patent Application Laid-Open No. 2002-152080 and M. Usami et al., “Powder LSI: An ultra small RF identification chip for individual recognition applications”, ISSCC Dig. Tech. Papers, February 2003, pp. 398-399.
However, to perform rectification of the diode, a voltage not less than the threshold (approximately 0.7 V) of the MOS transistor must be applied across the PN junction, i.e., across the source and drain. Therefore, conventional rectifier circuits cannot rectify the AC signal with an root-mean-square value less than the threshold. This means that the RFID tag cannot receive a weak signal transmitted by a reader/writer. Actually, such limitation of receivable signal power restricts the distance that the RFID tag can communicate with the reader/writer, to approximately 30 cm. This distance requires approach to the reader/writer of persons carrying the RFID tag or the RFID tag attached items, so that convenience is reduced. This distance also makes one reader/writer difficult to detect plural RFID tags simultaneously, and restricts the application range of the RFID tag.